Support is requested for a research program aimed at studying the mechanisms by which differentiated cells control the amounts and kinds of specialized products synthesized. Pure cell cultures of differentiated chick embryo chondrocytes will be used as a model system to study possible coregulation of two specialized products (collagen and chondromucoprotein) made by the same cells and possible feedback control of matrix production by extracellular materials. The experimental approach involves the use of several environmental factors, such as a cartilage conditioned medium factor and other extracellular materials, as tools to elucidate regulatory steps in the production of specific components of cartilage matrix.